148
FXUS65 KTWC 180915
AFDTWC

Area Forecast Discussion
National Weather Service Tucson AZ
215 AM MST Tue Nov 18 2025

.SYNOPSIS...A system moving into Southeast Arizona the middle of
the this week will bring cooler temperatures, chances for
precipitation including snowfall above 7000 feet in the Sky
Islands, and breezy conditions. High temperatures will remain 2
to 6 degrees below normal today, then fall 10 to 13 degrees below
normal by Thursday. The first widespread freeze is expected
Friday morning for the valleys to the south and east of Tucson.

&&

.DISCUSSION...The upper pattern this morning is dominated by a
amplified trough of low pressure digging south along the West
Coast with its associated low-center across central California.
The strong NLY flow on the backside of this feature will assist
this feature to continue to dig south today toward Southern
California. This upper low will bottom out near San Diego between
19/00Z-12Z this evening, stall while the low-center elongates
between Southern California and the Spur of the Baja by 20/00Z
Wednesday afternoon. While this is happening, a subtropical
moisture plume (with PWATs 1.00+ inch) along the SE periphery of
the upper-low will focus northward from the northern Gulf of
California into west central Arizona today, then central Arizona
tonight. The 18/00Z HREF indicated a 30 percent probability of
more than an inch of rain in the 3-hr QPF (ensemble probability
matched mean) in west central Arizona at 18/18Z, decreasing to 10
percent in the afternoon but briefly expanding south into western
Pima county by 19/00Z, then consolidating in west central Arizona
this evening and drifting west into central Arizona by 19/12Z
Wednesday morning.

Although this focused subtropical moisture plume will generally
spare Southeast Arizona from the heavier rainfall, we will start
to see precipitation spread across the majority of the area
Wednesday into Thursday as the upper low begins to ejects through
the area. The expectation is that the majority of the precipitation
will fall Wednesday afternoon into Thursday morning, with QPF
amounts between 0.33 and 0.67 inches. The Sky Islands and the
White mountains will see between 0.67-1.25 inches, with the
heaviest precipitation expected near the White Mounatins. The
18/00Z NBM probability of storm-total (72 hours) QPF in excess of
an inch is 5-10 percent across the majority of the valleys in
Southeast Arizona, and 30-50 percent in the northern Sky Islands
to the north and northeast of Tucson.

We are at that time of year now where we have to talk about snow
levels. They will this afternoon afternoon between 8500-9500
feet Wednesday afternoon, lowering to 6500-7500 feet Thursday
morning. Storm-total snowfall amounts between 2-5 inches will be
possible above 7500 feet in the Sky Islands with 4-7 inches
possible in the White Mountains. These snowfall amounts will
result in minor-to-moderate impacts for the higher elevations.

Otherwise, the main impact with this system will be the reinforcement
of cooler temperatures across Southeast Arizona. The 18/00Z
deterministic GFS and ECMWF both continue to lower the 850-700mb
thickness values to around 1550m by 00Z FRI, which would
correspond to a high temperature at the Tucson International
Airport (KTUS) around 56 degrees on Thursday. I suspect that we
wont be able to realize all of this cooling due to how rapidly the
upper low is ejecting, but an afternoon temperature near 60
degrees is likely none-the-less. The minimum temperatures Friday
morning will be more impactful. It looks like we will experience
the first widespread freeze this season in the eastern valleys.
The 18/06Z NBM probability of freezing temperatures (at or below
32 degrees) Friday morning; 76 percent for KOLS, 67 percent for
KSAD and 70 percent for KDUG. This may eventually require freeze
headlines as we move through the week.

Otherwise, with the mean trough in place to our west, expect yet
another upper-low to dive along the same path toward SRN CA/NRN
Baja by Saturday morning, then ejecting across our neck of the
woods next weekend. This would bring another round of valley
rain/mountain snow and reinforce the cold temperatures in place
across the area.

&&

.AVIATION...Valid through 19/12Z. SCT-BKN 6-10k ft AGL expanding
in coverage from the west of KTUS to BKN-OVC 4-8k ft AGL with
ISOLD-SCT -SHRA through 19/00Z, then expanding to BKN-OVC 4-8k ft
AGL across the rest of Southeast Arizona overnight with SCT -SHRA
through the rest of the period. SFC wind SLY less than 12 kts
through 18/17Z, then SWLY 15-20 kts with gusts to 25-30 kts in the
afternoon, decreasing to 10-12 kts overnight. The stronger winds
today will be S and SW of Tucson, impacting the KTUS and KOLS
terminals. Aviation discussion not updated for TAF amendments.

&&

.FIRE WEATHER...A storm system will begin to impact Southeast
Arizona this afternoon/evening across central and western Pima
county, then the rest of Southeast Arizona Wednesday into
Thursday. This system will bring numerous showers with a slight
chance of thunderstorms, snowfall to the mountains above 7000
feet, well below normal temperatures and breezy south to southwest
winds. Minimum RH values will be well above critical levels over
the next week, with values generally 35-45 percent in the valleys
and 65-75+ percent in the mountains.

&&

.TWC WATCHES/WARNINGS/ADVISORIES...None.

&&

$$

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